1. Field of the Invention
The present invention relates to a focusing state discriminating system and, more particularly, to a focusing state discriminating system which discriminates the focusing state of an image forming optical system, based on signals representing image formation states of images formed at first, second and third positions of the optical system which substantially correspond to positions slightly in front of, coincident with and slightly behind a predetermined focal plane of the image forming optical system.
2. Description of the Prior Art
Various systems have been proposed for discriminating the focusing state of an image forming optical system, based on signals representing image formation states of images formed at first, second and third positions of the optical system which substantially correspond to positions slightly in front of, coincident with and slightly behind a predetermined focal plane of the image forming optical system. Examples of such systems are disclosed in U.S. Pat. No. 3,532,045 (corresponding German DOLS No. 1,547,457); U.S. Pat. No. 4,227,156 (corresponding to German DOLS No. 2,923,573); and Japanese Laid-Open Patent Applications No. 147336/1976, No. 53-79,425, and No. 106421/1980.
Examining the methods of these five prior art examples for discriminating the focusing states, in the system of the first example, U.S. Pat. No. 3,532,045, the conditions for discriminating the in-focus state are:
f2&gt;f1 PA1 f2&gt;f3 PA1 f2-f1=f2-f3 PA1 f'2&gt;f'1 PA1 f'2&gt;f'3 PA1 P2&gt;P1 PA1 P2&gt;P3 PA1 P1=P3 PA1 f'2&gt;f'1 PA1 f'2&gt;f'3 PA1 .vertline.f'1-f'2.vertline.&lt; or .ltoreq. .alpha.f'2
where f1, f2 and f3 are signals representing contrasts of images at the first, second and third positions, respectively, as described above.
In the system in the second, U.S. Pat. No. 4,277,156, the in-focus state is detected by the peak of P2 in the range satisfying P2&gt;P1 and P2&gt;P3, where P1, P2 and P3 are output signals from photoelectric elements corresponding to the first, second and third positions as described above.
In the systems disclosed in the third and fourth, Japanese Patent Applications No. 147336/1976 and No. 53-79,425, the conditions for discriminating the in-focus state are:
where f'1, f'2 and f'3 represent the sharpness of the images at the first, second and third positions as described above.
Finally, in the fifth, Japanese Patent Application No. 106421/1980, the conditions for discriminating the in-focus state are:
where P1, P2 and P3 are defined as in U.S. Pat. No. 4,277,156.
In the first, second and fifth examples, the in-focus state is understood not as a range of finite value but as a single point. However, this is not very practical and may result in inconvenience when it is considered in terms of optical equipment such as general cameras. It is to be noted that the first and fifth examples build upon the same principle.
To the contrary, in the third and fourth examples, the in-focus state is discriminated within a relatively wider range than a single point. This, however, also results in other inconveniences such as focusing precision.
In order to eliminate the problems as described above, a system which sets a range for the in-focus state which is neither too wide nor too narrow is proposed in U.S. patent application Ser. No. 151,533, "Focus Detecting System", Sakai et al, filed on May 19, 1980 (corresponding German Patent Application No. P 30 19 901.0) by the assignee of the present application. The embodiment of this invention shows a system which defines the conditions for the in-focus state as:
(where .alpha. is a constant).
According to this system, the range for discriminating the in-focus state is defined by .alpha.f'2, that is, a function of a signal f' representing the image formation state of the image. The range for discriminating the in-focus state is less subject to adverse affects of fluctuations in brightness of the image or changes in contrast, resulting in an advantage of more precise and stable discrimination.